London Power JCM 800

[peteko]

Hello,

Im trying to build a London Power JCM 800 (for the fourth or fifth time... I like to learn the hard way I guess), and I'm having some trouble. Has anyone tried building this circuit, or verified that it works?

Here are some voltages that Im getting:

Va: 433V
-Vb:-28.1 (at the zener and about a volt less at pin 5 on the tube)

The latter seems rather low (this is with bias pots turned all the way low, as negative as possible).

Im thinking that my zener may be whacked. Or I'm dropping too much across the 15k before the zener. If you dont have the schem, I can try to explain better. Thanks for your time,
-Pete

[peteko]

Va Fluctuations

I meaured Va again today and it has changed to 450V. Does anyone have any idea why this would change 20 volts?? I haven't made any changes to the circuit.

Thanks,
Pete

[Mark Black]

Just a thought - could it be the mains voltage has changed? Around where I live it can vary by 5% or so depending on customer load, and that could account for the increase you have seen.

Mark

[peteko]

Mains

Hi Mark,

Thanks for the reply. I just checked the mains again, and since yesterday its down about 7 volts, so that explains the Va change. I have been playing around a bit with the bias pot values, etc, and found that if I increase their values to about 50k, I can get a more negative voltage at the control grid. Can you recommend a (negative) voltage range to shoot for?

Thanks again,
Pete

[Ray Ivers]

Pete,

Do you have a book and page reference for the circuit?

Ray

[peteko]

Yes, the book is TUT3 and the schematic is on pages 8-8 through 8-9 (The800 Chapter). Thanks,
Pete

[Ray Ivers]

Pete,

OK, thanks. I'd first pull your output tubes and disconnect the -Vb output to the bias pots, then measure your DC voltage to ground on each side of the 15K resistor and see what you get - then lift the Zener and measure again.

You're following the schematic religiously, or just using parts you have on hand?

Ray

[peteko]

Ray,

I am using the same values, but higher power ratings on some components (i.e., the 15k is a 3 watt resistor instead of a 1 watt. And the Zener is a 100v 5 watt diode). Also, I'm using switching jacks instead of a rotary for the OT.

Ok, so I disconnected the bias supply from the pots, and got some better readings. On the 1n4007 side of the 15k, i am reading 126.5v and on the Zener side (actually the drop on the Zener) is 100.5 v.

I have played around with the bias pot values a bit, and got more negative voltage on the grid with higher pot values. 100k gave too much and when I tried to bias, I couldn't get near the -30mV reading I was going for (it was too negative). 50k pots worked a bit better, but then I got to thinking that something may be wrong with a components somewhere because I don't think that there would be a mistake in the book. Of course there will be tolerance differences, so some tweaks might have to be made, but I'm still not sure about making the pot change. What do you think?

Thanks for your help,
Pete

[Ray Ivers]

Pete,

I am using the same values, but higher power ratings on some components (i.e., the 15k is a 3 watt resistor instead of a 1 watt. And the Zener is a 100v 5 watt diode).

FWIW, I'd do the same. OTOH, I've seen switching jacks go intermittent sometimes - not usually a big deal in an input jack, but a speaker jack... well, YMMV.

Ok, so I disconnected the bias supply from the pots, and got some better readings. On the 1n4007 side of the 15k, i am reading 126.5v and on the Zener side (actually the drop on the Zener) is 100.5 v.

Exactly what I thought would happen - it appears to me your bias supply (up to & including the Zener) is "fine" - but please keep reading. .

I have played around with the bias pot values a bit... but I'm still not sure about making the pot change. What do you think?

If you change the pot values, you'll have to change the other resistor values as well, to keep the pot adjustment range roughly the same. If it were me I would put back in the 25K pots, use the resistor values on the schematic, and make the change outlined below.

I just did the math, and Kevin's supply (using the specified 100V, 1W Zener, and assuming -126V pre-resistor -Vb) will allow only 1.7mA of Zener current, meaning the Zener could only supply a bit less than this, maybe 1.6mA or a little more... but the bias circuit pots & associated components draw at least 8.33mA of current just by themselves, assuming no grid-conduction current or component/AC-line variations.

Using a 5W Zener (as you are, and which is what I would do) this is no longer a problem, so... assuming @ -126V raw bias under load, try replacing the 15K 3W with a 1.5K 2W or 3W resistor; this will allow about 17mA Zener current, which should be more than enough for practically any contingency short of a failure mode, using the original 25K bias pots. 1.5K Rdiss will be @ .5W, and 100V 5W Zener dissipation about 1.73W.

Ray

[peteko]

Hi Ray,

I will try the replacement. When you calculate the current draw of the bias pot and resistors, are you just counting the 25k pots, the 220k resistors from the " o " lug to the wiper (on the bias pot), and the 6k8 resistor to ground from the bias pot " x " lug? Without the tubes, the grid stop resistor doesn't come into play and the 22n caps block dc.

So at one extreme of the bias pots we have 220k in parallel with 25k and at the other end, we have just 25k because the 220k is shorted, right? Thanks for your time,

Pete

[Ray Ivers]

Pete,

I used 12K for my approximate Zener load; four 25K pots in parallel, in series with the 6.8K resistor to ground; I see now that the actual total load is @ 13K... so the bias-adjustment-circuit current draw will be about 7.7mA, not the 8.33mA I posted - not a massive difference IMO.

I didn't include the 220K 'pot-failure safety resistors', they will increase overall bias-circuit current draw slightly (from 7.7mA to @ 8mA) if all were set to the minimum-bias-voltage setting, which I figured was unlikely - and my suggestions more than cover this possibility anyway.

My original recommendations should still work fine. The idea is to build a bias supply that can supply more current than needed, so that you don't have to worry about 500uA here, a couple of mA there, etc.

Ray

[peteko]

Hi Ray,

With the 1.5k I get -31.1mV at the grid, much better (I'm shooting for about 27mA on the grid). However here's the next problem, when I plug in the el34's, the current begins to rise through the grid, and keeps rising past 27mA, and I'm turning the amp off when it gets to 50mA. Do you think this is due to faulty tubes? (I may have fried these in previous attempts).

Pete

[Ray Ivers]

Pete,

With the 1.5k I get -31.1mV at the grid, much better (I'm shooting for about 27mA on the grid).

I assume here you mean 'cathode' instead of 'grid' - TP1 thru TP4 on page 8-9 of TUT 3. What negative voltage do you read at pin 5 of each output tube socket?

when I plug in the el34's, the current begins to rise... and keeps rising past 27mA, and I'm turning the amp off when it gets to 50mA. Do you think this is due to faulty tubes? (I may have fried these in previous attempts).

If you're reading a ballpark-correct negative bias voltage at each output socket pin 5 (@-34V to -45V, but YMMV), I would leave your meter connected to one pin 5 and plug in one output tube at a time in that socket - you may find one or more of them start(s) to 'run away' and pull the bias voltage down, allowing the current to rise, etc., and may have to only replace that one (although if you think they're all damaged, I would replace them all - it's hard enough to troubleshoot a new build without having tube problems to contend with).

Ray

[peteko]

Hi Ray,

Sorry for the confusion, I got my terms mixed up. I am reading -31.1V at the grid (pin 5), in retrospect still a bit low.

The cathode current is running away (test points). It's here that I want the 27mA, but the current keeps rising way past 27mA (up to 100mA before I could turn it off).

Pete

[Ray Ivers]

Pete,

I am reading -31.1V at the grid (pin 5), in retrospect still a bit low.

You should be able to adjust the voltage at pin 5 from a bit less negative than -100V to a bit less negative than -30V.

The cathode current is running away.

What does the bias voltage at pin 5 do during runaway?

Ray